<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>

</body>
<script>
    alert(typeof 3);
    alert(typeof "a");
    alert(typeof true);
    alert(typeof null);

    var a;
    alert(typeof a);


</script>
</html>